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Monte Carlo studies of model Langmuir monolayers

Description

Citation

Title

Monte Carlo studies of model Langmuir monolayers

Author(s)

Sheldon B. Opps, B. Yang, C. G. Gray, D. E. Sullivan

Journal

Physical Review E

Date

2001

Volume

6304

Issue

4

Start page

041602

End page

041602

Abstract

This paper examines some of the basic properties of a model Langmuir monolayer, consisting of surfactant molecules deposited onto a water subphase. The surfactants are modeled as rigid rods composed of a head and tail segment of diameters sigma (hh) and sigma (tt), respectively. The tails consist of n(t)approximate to4-7 effective monomers representing methylene groups. These rigid rods interact via site-site Lennard-Jones potentials with different interaction parameters for the tail-tail, head-tail, and head-head interactions. In a previous paper, we studied the ground-state properties of this system using a Landau approach. In the present paper, Monte Carlo simulations were performed in the canonical ensemble to elucidate the finite-temperature behavior of this system. Simulation techniques. incorporating a system of dynamic filters, allow us to decrease CPU time with negligible statistical error. This paper focuses on several of the key parameters, such as density, head-tail diameter mismatch, and chain length, responsible for driving transitions from uniformly tilted to untilted phases and between different tilt-ordered phases. Upon varying the density of the system, with sigma (hh) = sigma (tt), we observe a transition from a tilted (NNN)-condensed phase to an untilted-liquid phase and, upon comparison with recent experiments with fatty acid-alcohol and fatty acid-ester mixtures [M. C. Shih, M. K. Durbin, A. Malik, P. Zschack, and P. Dutta, J. Chem. Phys. 101. 9132 (1994): E. Teer, C. M. Knobler, C. Lautz, S. Wurlitzer, J. Kildae, and T. M. Fischer, J. Chem. Phys. 106. 1913 (1997)], we identify this as the L-2'/Ov-L-1, phase boundary. By varying the head-tail diameter ratio, we observe a decrease in T-c with increasing mismatch. However, as the chain length was increased we observed that the transition temperatures increased and differences in T-c due to head-tail diameter mismatch were diminished. In most of the present research, the water was treated as a hard surface, whereby the surfactants are only allowed to move within the plane of this surface. However, we have also utilized a more realistic model for the surfactant-water interactions, developed by Karaborni and Toxvaerd, in order to examine the role which the coupled effects of head group size and head group-subphase interactions plays in determining tilt ordering and on the stability of the monolayer. It is found that increasing the head diameter results in a widening of the air-water interface and an associated destruction of orientational order. Furthermore, the onset of capillary waves at lower temperatures for larger head diameters implies that the L-2-L-1 phase boundary for acids and acetates should move to lower temperatures relative to the L-2'/Ov-L-1 phase boundary for alcohols acid esters. This feature has yet to be seen in experimental studies.